1. Field of the Invention
The present invention relates to a vacuum metallized polyimide film. More specifically, the invention relates to a vacuum metallized polyimide film comprising an aromatic polyimide layer and a metal layer bonded integrally, giving high bonding strength and high adhesion after exposure to temperature and humidity aging. The metal layer is formed without the aid of an adhesive by vacuum deposition of a thin metal layer followed by electroplating to the desired metal thickness. The metallized polyimide film can be utilized for various purposes, such as for flexible printed circuits and multilayer printed wiring boards, rigid-flex construction, and tape automated bonding, as well as for heaters, antennas and anti-static films.
2. Description of the Prior Art
Certain electronic assemblies have conductor traces on one or both sides of a dielectric substrate. Preferred dielectrics for applications requiring flexible substrates, especially in multilayer constructions, and tape automated bonding (TAB) involve the use of polyimide films. In general, the conductive layers on the polyimide film substrate are provided through the use of metal foils and adhesives especially formulated for their physical and thermal stability. The conductive layers are also provided, in some cases, through direct metallization by vacuum deposition or by electroless deposition methods well-known in the art.
The addition of small amounts of hydrocarbyl tin compounds and tin salts into the polyimide film has recently been disclosed in U.S. Pat. Nos. 5,218,034 and 5,272,194, as improving adhesion when the polyimide film is bonded to metal foils using an epoxy, acrylic or other heat-resistant adhesive. There is, however, no teaching or suggestion of the surprising and unexpected adhesion improvement after exposure to temperature humidity aging provided by a metallized polyimide film formed by directly bonding a polyimide film layer containing a hydrocarbyl tin compound to a metal vacuum deposited layer without the use of an adhesive, as specifically claimed herein.
The adhesive bonding method has certain disadvantages, especially in free line applications, multilayer applications, and applications requiring chemical milling, where either the properties of the adhesive or the physical space occupied by the adhesive are limiting factors. For example, adhesively bonded polyimide film-metal laminates can exhibit poor dimensional stability, a severe disadvantage for laying up multilayer boards; or limited resolution, a disadvantage in making fine line circuits.
A preferred construction, especially in multilayer applications and fine line circuits such as TAB, would avoid the use of adhesives altogether and provide the metal directly bonded to the polyimide film substrate. However, the durability of directly metallized polyimide film substrates also poses significant problems.
Two methods for preparing adhesiveless metal coated polyimide films are the related methods of evaporative and sputter deposition, followed by electrolytic copper buildup to obtain the desired thickness. Sputtering generally provides better adhesion than evaporative deposition, but clads made with both techniques exhibit significant loss of adhesion when exposed to temperature humidity aging.
In order to improve the adhesive strength between the aromatic polyimide film and the metal layer, as described above, attempts have been made to roughen the surface of the polyimide film, or to introduce reactive functional groups by a corona discharge or plasma treatment, or by a chemical surface treatment. Nevertheless, even when using such known methods, loss of adhesion occurs during temperature humidity aging. Thus, a need still exists for a metallized polyimide film, which eliminates the aforesaid disadvantages, and which has a high adhesive strength between the polyimide film layer and the vacuum deposited metal layer.